Like a call made using a mobile telephone, with speech communication in a mobile communication system, currently, communication using a monaural scheme (monaural communication) is mainstream. However, hereafter, like a fourth generation mobile communication system, if the transmission rate becomes a still higher bit rate, it is possible to ensure a bandwidth for transmitting a plurality of channels, so that it is expected that communication using a stereo scheme (stereo communication) will be also spread in speech communication.
For example, when the current situation is considered where the number of users increases who enjoy stereo music by recording music in a mobile audio player provided with a HDD (hard disc) and attaching earphones or headphones for stereo to the player, in the future, it is predicted that mobile telephones and music players will be linked together and a life style will be prevalent where speech communication is carried out using a stereo scheme utilizing equipment such as earphones and headphones for stereo. Further, in an environment such as video conference that has recently become widespread, in order to enable conversations having high-fidelity, it is predicted that stereo communication is performed.
On the other hand, in a mobile communication system and wired communication system, in order to reduce load of the system, it is typical to achieve a low bit rate of transmission information by encoding speech signals to be transmitted in advance. As a result, recently, a technique for coding stereo speech signals attracts attention. For example, there is a coding technique for increasing the coding efficiency for encoding predictive residual signals to which weight of CELP coding for stereo speech signals is assigned, using cross-channel prediction (refer to non-patent document 1).
Further, even if stereo communication becomes widespread, it is predicted that monaural communication will be still carried out. This is because monaural communication is performed at a low bit rate, and therefore it is expected that communication costs will decrease. Further, the circuit scale of mobile telephones supporting only monaural communication is small, and therefore such mobile telephones are inexpensive. Users that do not desire high-quality speech communication may therefore purchase mobile telephones supporting only monaural communication. As a result, in one communication system, there may be a mixture of mobile telephones supporting stereo communication and mobile telephones supporting monaural communication. It is therefore necessary for the communication system to support both stereo communication and monaural communication. Further, in a mobile communication system, communication data is exchanged using radio signals, and therefore there are cases where part of the communication data may be lost according to a channel environment. It is therefore extremely useful for mobile telephones to have a function capable of restoring original communication data from the remaining received data even if part of the communication data is lost.
As a function of supporting both stereo communication and monaural communication, and capable of restoring original communication data from the remaining received data even if part of the communication data is lost, there is scalable coding consisting of a stereo signal and a monaural signal. An example of the scalable encoding apparatus having this function is as disclosed, for example, in non-patent document 2.
Non-Patent Document 1: Ramprashad, S. A., “Stereophonic CELP coding using cross channel prediction”, Proc. IEEE Workshop on Speech Coding, Pages: 136-138 (17-20 Sep. 2000)
Non-Patent Document 2: ISO/IEC 14496-3:1999 (B.14 Scalable AAC with core coder)